Patent GB-1497194-A (GB)
Title: Transistor amplifier
IPC: H03F1/34
Filed: 1975-07-01
Abstract: 1497194 Transistor amplifiers RCA CORPORATION 1 July 1975 [5 July 1974] 27702/75 Heading H3T A transistor amplifier, particularly if integrated, in which differential input potentials at 101, 102 are acceptable if in the neighbourhood of a reference level (earth), swinging both above and below, comprises differentially coupled input PNP transistors 105, 106 with collectors directly connected to the collectors of a pair of complementary transistors 131, 132, these having common bases fed via common-mode feedback loops which maintain the collector-emitter potential of transistors 131, 132 below their base-emitter potential but above saturation level, and the input terminal quiescent level near earth potential. The commonmode loops run from the collectors of 105, 106 through potential shift stages 107-109 and 108-110 and through differential NPN output stage 111, 112 with NPN transistor 133 as emitter load; taking the base-emitter voltage of each transistor as a uniform value (0À65 V), and that across Schottky diodes 109, 110 as half this, the loops provide half a base-emitter offset potential at the collectors of 131, 132 and zero at the bases of 105, 106. The loads 123-128 may be resistive or active devices forming constant current sources, 125, 126 and 127, 128 being respectively identical. If single ended output is required load 128 may be shorted or an equal current may be forced to follow that through load 127 by a current-mirror amplifier. Other modifications of the common-mode loops, still maintaining collector potential on 131, 132 at half the base potential and inputs near earth, are described, the differential-mode in each case being independent of the common mode feedback. If the Schottky diodes 109, 110 are replaced by junction diodes or diode connected transistors, with drop equal to a base-emitter potential (Fig. 2, not shown), transistor 133 is replaced by a resistive potential divider passing to the bases of 131, 132 two thirds of the potential above earth of the emitters of 111, 112. If the input swing is high, differential mode feedback may be introduced by putting all or part of the upper limb of the divider in the individual 111, 112 emitter feeds (Fig. 4, not shown). The output current through 111, 112 is limited by diode-connected transistor 133 to a level depending on the transistor area under baseemitter potential equal to that on 131, 132. If the transistor 133 is replaced by a constant current sink with the emitters of 111, 112 coupled via an emitter follower to the bases of 131, 132 (Fig. 3, not shown), the loads 125, 126 may be replaced by emitter follower stages having base-emitter potential set by a series pair of junction diodes each dropping one base-emitter potential; emitter loads and the resistors replacing the Schottky diodes 109, 110 are chosen to drop 1À0 and 1À5 respectively times the base-emitter potential, the resistors being calculated taking α = 0À9 relating collector to emitter current in the emitter-followers. The output transistors 111, 112 may be removed from the commonmode feedback loops (Fig. 5, not shown) by providing a supply (as in Fig. 3, not shown) but commoned to feed both transistors 107 and 108 via emitter loads dropping half a base-emitter potential, the emitter follower (as in Fig. 3, not shown) supplying the bases of 131, 132 now having its base fed from the common point of these two emitter loads.